1-octene, which is a monomer or comonomer for producing linear low-density polyethylene, is a commercially important raw material which is widely used in a polymerization process and is used as a specific drug.
Higher α-olefins, necessary for producing linear low-density polyethylene, are obtained through an ethylene oligomerization reaction. However, the ethylene oligomerization reaction is inefficient in that a large amount of butene, octene, derivatives of octene and specific higher oligomers are formed together with polyethylene.
In conventional ethylene oligomerization technologies, generally, various α-olefins are formed depending on the Schulz-Flory or Poisson product distribution, and thus the yield of desired products is limited. In relation to this, U.S. Pat. No. 6,184,428 discloses a nickel-based catalyst comprising 2-diphenyl phosphino benzoic acid as a chelate ligand, NiCl2.6H2O as a nickel precursor, and sodium tetraphenyl borate as a catalyst activator. It is also disclosed in this patent document that, in the oligomerization of ethylene using the nickel-based catalyst, the selectivity of 1-octene is 19%.
Further, German Patent No. 1,443,927 and U.S. Pat. No. 3,906,053 disclose Ziegler catalysts produced based on a trialkyl aluminum catalyst. It is also disclosed in these patent documents that 13˜15 wt % of 1-octene, based on the total amount of an olefin mixture, can be produced using the Ziegler catalyst.
Recently, research on methods of producing 1-octene by selectively tetramerizing ethylene through transition metal catalysis has been conducted. Here, most commonly-known transition metal catalysts are chromium-based catalysts.
Recently, it was disclosed in WO 04/056479 that 1-octene is produced by tetramerizing ethylene using a chromium-based catalyst including a hetero atom ligand having phosphorus and nitrogen as heteroatoms. Here, examples of the heteroatom ligand, which is used for an ethylene tetramerization catalyst, may include (phenyl)2PN(isopropyl)P(phenyl)2 and the like.
It is also disclosed in the above conventional technology that 1-octene can be produced at a selectivity of more than 70 wt % by tetramerizing ethylene using a chromium-based catalyst including a heteroatom ligand having phosphorus and nitrogen as heteroatoms without substituents that are polar to hydrocarbyl groups or heterohydrocarbyl groups, which are bonded with phosphorus.
However, conventional technologies are problematic in that, in relation to the structure of ligands including heteroatoms, specifically, they cannot clearly demonstrate the highly selective production of 1-octene by tetramerizing ethylene when catalysts include what kind of ligand, in that they can present only a PNP backbone structure ligand, for example, (R1)(R2)P—(R5)N—P(R3)(R4), as a ligand having a 1-octene selectivity of about 70 wt %, and in that the structure of the substituents of the ligands including heteroatoms is also limitedly disclosed. Further, the conventional PNP backbone structure ligand having heteroatoms is also problematic in that, in the preparation of 1-octene, reaction activity cannot be maintained constant, and the reaction rate is also rapidly decreased with the reaction of time.